Here is a soft robot that can curl into a pentagonal wheel when placed on a hot surface. The Rollbot is untethered and measures 8cm long and 4cm wide. As Harvard researchers explain:
Another set of hinges is embedded on each of the five sides of the wheel. A hinge folds when in contact with the hot surface, propelling the wheel to turn to the next side, where the next hinge folds. As they roll off the hot surface, the hinges unfold and are ready for the next cycle.
These types of robots can be programmed to respond to light, pH, humidity, and other external stimuli too.